ANATOMY OF PALUDESTRINA 181 



teriorly, lined with columnar ciliated cells with more or less 

 basal nuclei. The rest of the structure of this gland, which 

 stains violet with haematoxylin and eosin, is not unHke that of 

 the lighter-staining portion of the accessory gland of the 

 female. 



Below the prostate the vas deferens becomes smaller, thick- 

 walled, and ciliated. It eventually runs just below the 

 epidermis in the floor of the pallial cavity to the penis, which it 

 traverses up to its apex. The penis is single in contrast 

 with the remarkable complexity of Bythinella and 

 Bythinia (Moquin Tandon, 13), in which a flagellum and 

 a second branch occurs. It therefore exhibits the condi- 

 tion seen in Cyclostoma (Garnault) and Vitrella. In 

 P. ulvae the penis is quite simple according to Henking, 

 while drawings made from the living animal by my friend 

 Dr. H. Quick also show no accessory structures upon the male 

 organ. The intromittent portion in P. ventrosa is long 

 and pointed. 



3. Habits, etc. 



A preliminary attempt has been made (Eobson, 17) to 

 analyse the ecological conditions under which P. ventrosa 

 is found. But a great deal remains to be done upon this subject 

 as well as upon the distribution and ecology of the plants 

 associated with it and upon which it may be presumed to 

 depend. Though a more definitely brackish-water form than 

 P. ulvae, the case worked out at Leigh-on-Sea demonstrated 

 a greater adaptability and tolerance on the part of P. ven- 

 trosa. If, as we may rightly assume, the British Paludes- 

 trinidae show a progressive tendency to become adapted to 

 fresh-water, P. ventrosa represents an intermediate stage 

 of adaptation, but exhibits the tendency in its initial rather 

 than its later stages. Little can be said upon the more intimate 

 habits of this animal. It is usually found upon some water- 

 plant, but quite frequently upon mud or bottom debris. In 

 several examples from Leigh, Wakering Wick, and elsewhere, 

 the stomach contained a variety of diatoms and a few foramini- 



